The conventional method of treating sewage waters is based on the removal of materials contained in the sewage water by oxidation. For the nitrogeneous load materials, for example, this involves conversion to nitrate. To this end, the prior art large scale methods, for example the activated sludge process, the biological filter process, bio-oxidation ponds and the like are used. In these plants nitrification is effected using ubiquitous nitric bacteria in accordance with the reactions EQU NH.sub.4.sup.+ +1.5O.sub.2 =NO.sub.2.sup.- +2H.sup.+ +H.sub.2 O+0.66 kcal (nitrosomonas) EQU and EQU 2NO.sub.2.sup.- +O.sub.2 =2NO.sub.3.sup.- +17.5 kcal (nitrobacter)
As a particularly inexpensive variant, the aerated clearing pond with removal of the materials contained in the sewage water is preferred for some time. Unfortunately the nitrification capacity of such ponds is sometimes very poor.
The more recent developments of sewage purification technology to a large extent involve removal of compounds difficult to be degraded and the removal of the nutrients nitrogen and phosphate in addition to the conventional elimination of the organic substances easily to be degraded. This concept of the process is in conformity with the farther-reaching demands to keep lakes and rivers pure and with the more severe legal regulations resulting therefrom for the passage of purified sewage water into lakes or rivers.
Such a process technology is generally called advanced sewage treatment. Numerous technological developments have been initiated in this respect, and a large number of processes have been suggested. Thus precipitation reactions are used for the elimination of phosphate. In addition, a flocculus rich in phosphate is caused to precipitate in precipitation equipment by adding iron, aluminum and calcium compounds and is separated from the flow of sewage water; see, for example, "Lehr- und Handbuch der Abwassertechnik" Vol. IV: Biologisch-chemische und weitergehende Abwasserreinigung, publisher Ernst & Sohn, Berlin 1985.
At the same time, efforts are made to combine nitrification and subsequent denitrification for the elimination of nitrogen by appropriate technical arrangement. To this end, aerobic and anoxic phases separated in space have to be provided in the purification system and have to be brought into appropriate contact. In many cases, such arrangements prove particularly effective in the removal of compounds difficult to be degraded. For the denitrification as a subsequent process to the nitrification, compartimental plants have been built, which have anoxic areas due to a control of the oxygen gradient, or, after thorough oxidation of the sewage water, the developed nitrate is reduced under anoxic conditions, an electron donator (methanol, molasses) having been added anew; see, for example, "Lehr- und Handbuch der Abwassertechnik," l.c.
As in recent times the removal of ammonia and of its derivates from sewage water has been attached great importance to, at present particular efforts to solve the nitrogen problem in satisfactory manner can be observed. These efforts partially extend already to improved conversion of the ammonia to the substantially less toxic nitrate. However this process does not run in satisfactory manner in large scale sewage treatment plants, thus, for example, in aerated pond plants. Attempts are made here to support nitrification by the construction of additional technical equipment (fixed bed reactors); see, for example, "Lehr- und Handbuch der Abwassertechnik," l.c.
On the other hand, an advanced concept of the process is to remove the nitrate in anoxic phases. With activated sludge plants, denitrification can be carried out in separate tanks. Alternatively simultaneous nitrification and denitrification can be carried out in one tank. However this requires a correspondingly complex control of this advanced sewage water purification. See in this respect for example "Lehr- und Handbuch der Abwassertechnik", l.c.
It has been well known for a long time that appropriately prepared ground bodies in particular planted ground bodies, can be adapted to achieve the advanced sewage water treatment mentioned in the beginning, if certain conditions are met. The land treatment processes quite effective in this respect have been supplemented, in recent times, by additional processes, which use ground bodies or fixed mineral beds and provide considerably higher capacities, see R. Kickuth: "Abwasserreinigung in Mosaikmatritzen aus aeroben und anaeroben Teilbezirken" in F. Moser (editor), "Grundlagen der Abwasserreinigung", Schriftenreihe Wasser-Abwasser 19, Munchen/Wein 1981, page 639 ff.; R. Kickuth: "Verfahren zum Aufbau definierter Phosphatdepots aus Abfallphophaten, European Pat. No. 0028360 of 4/20/83. It could be shown, for example, that planted fixed mineral beds can achieve, besides a noticeable reduction of organic loads, also a considerable elimination of the nutrients phosphate, nitrogen and sulfur. Partially also noticeable elimination capacities with heavy metals and refractory carbon compounds are achieved. Problems may be presented bu such sewage treatment plants due to their
(a) rather large area of ground required, PA1 (b) their heavy dependency on atmospheric conditions, for example impeding of the nitrification at low outside temperature, PA1 (c) their relatively low hydraulic capacity.
Thus also this process, which is very advantageous with regard to construction and operating costs and very efficient with regard to capacities, is not without problems, see Kraft: "Zur Problematik der Konstruktion von Sumpfpflanzenklaranlagen"; Korrespondenz Abwasser, Vol 1 (10), pages 840-846.